jaw, and they were distinguished as a genus Hexaprotodon, con-
trasting with Teiraprotoclon, until intermediate conditions were
observed. Choeropsis, as already observed, was a still further
reduction of the tetraprotodont type. The molars (the formula is
Fm f M §) when worn show a double trefoil pattern. The
orbital cavity is encircled by bone. As with many other aquatic
mammals the kidneys are lobulated.
FIG. 140.—Hippopotamus. Hippopotamus amphibivts. x ^.
A very singular fact about the Hippopotamus is the pro-
duction of a ** bloody sweat/* a carmine-coloured secretion, con-
taining small crystals and corpuscles,, from the skin. This
coloured fluid has of course nothing to do with blood.1
The animal grows to a length of at any rate 14 feet. The
limbs and the tail are short. Lake other aquatic animals the
nostrils are on the surface of the head, and can be closed when
the animal is under water. When it reaches the surface of the
water after a prolonged immersion, it spouts like a "Whale. Sir
Samuel Baker says that ten. minutes is the longest time that the
Hippopotamus caa remain below the water. It is frequently a
dangerous animal to encounter, as it will capsize boats, and even
Mte large pieces out of their bottoms; with, its huge teeth it
JRroc* Z<wl» See. 185O, p. 160.